1. Technical Field
This invention relates generally to an ion optical instrument, such as a mass spectrometer, combined GC-MS or LC-MS device, or any portion of such a instrument or device, and more specifically to a method and apparatus for pumping from one or more chambers in such a device or instrument.
2. Background
A typical mass spectrometer utilized for GC/MS requires some means of high vacuum pumping. This is necessary primarily for two reasons. The first reason is to remove permanent gasses such as nitrogen, oxygen and carrier gasses such as hydrogen or helium in order to achieve appropriate mean free path lengths for transmission of ion beams. Removal of such gasses additionally prevents unwanted ion-molecule reactions, oxidation of source components and high voltage breakdown. The second reason in maintaining a high vacuum environment is to remove introduced contaminants which would otherwise result in adverse analytical performance. Such adverse performance may include premature degradation in sensitivity or isobaric interference with signal. The introduced contaminants may include sample or matrix molecules, solvent molecules, buffer gasses, reagent gasses, oils from fingerprints, outgassing of plasticizers from polymeric components and the like.
A general configuration useful for the removal of these contaminants involves a turbomolecular pump backed by a suitable roughing pump. Often, multiply pumped systems using more than one turbomolecular pump, or a split flow arrangement are desired due to higher gas loads or a requirement for various sections of the vacuum manifold to operate at different pressures.
In a 1978 article of Analytical Chemistry, Vol. 50, No. 2 by L. P. Grimsrud shows and describes a diffusion pump in combination with a mass spectrometer vacuum chamber having a curtain that divides the chamber into two sections. The curtain is formed of two or three pieces of stainless steel, a baffle, and a butterfly valve. The curtain provides a modest amount of pressure differential during pumping. However, the Grimsrud's description is directed to a diffusion pump, which does not have rotors. As such, Grimsrud's disclosure is lacking in disclosure regarding positioning any elements in the chamber relative to a rotor.
U.S. Pat. No. 7,001,491 to Lombardi et al. has vacuum processing chambers for vapor deposition processing of silicon wafers. Lombardi teaches shielding of processing chamber surfaces and the maintenance and control of vacuum and gas flow in the vacuum processing chambers, at least in part, by shields. Thus, by use of the shields, separate pumps may not be necessary for one or more of the chambers since the shields create pressure differentials.